Magnetic Random Access Memory (“MRAM”) is a non-volatile memory that is being considered for long term data storage and is one exemplary type of resistive cross point memory. It is conceivable that other forms of RXPtM's will be developed that are not based on MRAM use, and the methods described in this invention disclosure will apply to those as well. A typical MRAM device includes an array of memory cells. Word lines extend along rows of the memory cells, and bit lines extend along columns of the memory cells. The memory cells are each located at a cross point of a word line and a bit line, and each memory cell includes two masses of magnetic material. One of the masses is magnetically fixed and the other is magnetically variable. A memory cell stores a bit of information as the orientation of relative magnetization of the fixed and variable materials. In other words, the magnetization of the each memory cell at any given time assumes one of two stable orientations. These two stable orientations, referred to as “parallel” and “anti-parallel” magnetic orientation, represent logic values of “0” and “1,”for example. The resistance of a memory cell varies dependent upon whether it stores a “0” or a “1” value. That is, the resistance of a memory cell is a first value “R” if the orientation of the magnetization of the fixed magnetic material and of the variable magnetic material is parallel, and the resistance of the memory cell is increased to a second value R+ΔR if the orientation of the magnetization is anti-parallel. The orientation of the relative magnetization of a selected memory cell (and, therefore, the logic state of the memory cell) may be sensed by sensing the resistance value of the selected memory cell.
Performing sense and write operations in MRAM devices could be orders of magnitude faster than performing sense and write operations in conventional long term storage devices, such as hard drives, for example. In addition, the MRAM devices could be more compact and could consume less power than hard drives and other such conventional long term data storage devices.
However, sensing the resistance state of a single memory cell in an array (and thereby “sensing” the data value) can be unreliable. All memory cells in the array are coupled together through many parallel paths (i.e., the bit and word lines). The resistance seen at one cross point equals the resistance of the memory cell at that cross point in parallel with resistances of memory cells in the other rows and columns (again, the array of memory cells may be characterized as a cross point resistor network).
There is a need to reliably sense the resistance states of memory cells in MRAM devices.
Currently, it is known to use a sense amplifier to sense a resistance value associated with a selected memory cell of an array. However, determining when the sense amplifier has an acceptable calibration or needs to be recalibrated is conventionally performed off of the chip on which the array of memory cells is fabricated. Further, conventional methods of determining when a new sense amplifier recalibration is required and performing that recalibration destroys data in a memory cell. In essence, calibration of such a sense amplifier is a laboratory procedure.
Further, calibration of a sense amplifier so that it can reliably perform this sense operation compensates at the same time for two aspects of the RXPtM array. These two aspects may be considered as “global” and “environmental.” That is, the sense amplifier is compensated or calibrated for global factors of the memory cell array that are constant over time. These global factors include such influences as process and geometry variations (i.e., asymmetries in the circuit design and fabrication non-uniformity resulting in threshold voltage variations and resistance and capacitance variations, for example) that occur during fabrication of the memory cell array. At the same time, the environmental factors then existing for the RXPtM array are compensated for. However, compensation for the global factors which are constant over time does not address needed compensations for environmental parameters which change over time. These environmental parameters include such factors as operating temperature of the RXPtM array, and power supply voltage variations.
Thus, there is a need to provide a method to determine when recalibration of sense amplifier offset (i.e., calibration) values is necessary for reliably sensing stored data values in a RXPtM.
Further, there is a need for providing a method to determine when recalibration of a sense amplifier is needed before data is lost because of an amplifier “out of calibration” condition.
Also, there is a need for such a sense amplifier recalibration to be performed without loss of data stored in a RXPtM array.
Still further, there is a need to have such a method and apparatus implemented on the same chip as the RXPtM cell array.